It is often necessary to apply coating materials to paper and, in some cases, to apply different coating materials to both major surfaces of the paper. For example, in the production of repositionable notes, such as the Post-It.RTM. brand note pads available from Minnesota Mining and Manufacturing Company, it is known to apply a primer material to one side of the paper from which the repositionable notes will be cut, and to apply a low adhesion backsize, or release, material to the other side of the paper. Repositionable adhesive is then applied to the paper on top of the primer material. Conventionally, for the production of repositionable notes, the various coatings are applied to a web of paper drawn from a continuous roll. The coating materials are dispersed in solvents and coated directly onto the paper web. The web is dried between coatings and then rewound, with the coated roll subsequently cut into sheets which are used to produce the notes.
A process for the production of repositionable notes, in which a release material and a primer material are coated successively on opposite sides of a paper web, is described in WO-A-87/05315.
In some cases, it is desirable to apply coating material to cut sheets rather than to a continuous web of paper. For example, in the production of repositionable notes it is often desirable to have the option of using a stack of preprinted sheets as the supply source, instead of a plain paper web, to extend the flexibility of the production process. In addition, for environmental reasons, there is a desire to move away from the use of environmentally destructive organic solvents in such coating processes, and towards more environmentally friendly water-based materials. It is moreover noted that many inks are soluble in organic solvents, but insoluble in water.
WO 94/19419 discloses an apparatus and a method for forming pads of repositionable notes from a stack of uncoated individual paper sheets. The sheets are fed from the stack in an overlapped condition to a coating station in which a continuous layer of a water-based primer material is applied to one major surface of the pseudo web of overlapped sheets, and a continuous layer of a water-based low adhesion backsize (LAB) material is applied simultaneously to the other major surface. The overlapped sheets are then dried and fed to a second coating station in which stripes of repositionable adhesive are transferred from an endless transfer belt to the pseudo web of overlapped sheets onto the surface to which the primer was applied in the first coating station. Thereafter, the sheets are adhered together in a stack and trimmed to form pads of repositionable notes.
Coating of Individual Sheets
In certain coating processes, it may be preferable for sheets to be coated individually rather than in the form of an overlapped pseudo web. However, commercial coating stations are generally designed for coating a continuos web of paper dispensed from a large roll, and cannot accommodate individual sheets.
Hence, efforts continue to develop a commercially viable system that will enable the coating of individual sheets with an effective amount of coating material.
Reversing Direction of Overlap
In certain circumstances, the handling of overlapped individual sheets can be facilitated by reversing the direction of the overlap as the sheets pass through certain segments of the coating process. When such a reversal in the direction of overlap is desired, the apparatus used to achieve the reversal should function reliably for a wide range of sheet sizes, weights and types.
It has been found that existing systems for applying a coating material to sheets, while having their own utility, are not as effective and flexible as desired. It has also been found that existing systems which use an endless transfer surface for applying a coating material to sheets commonly encounter problems in removing the sheets and the coating material from the transfer surface when certain types of coating materials and/or certain types and sizes of sheets are being coated. Therefore, an improved method and apparatus for applying coating materials onto sheets, including an improved method and apparatus for transferring a coating material from an endless transfer surface to sheets, is desired.